Often airway requirements force airplanes to change flight level while enroute. Some of these changes are part of route description, and are required for route to pass the validation process. For instance, in absence of high level airways leading to your final destination, you might want to descend earlier to a low level airway. This article explains how altitude changes are encoded in route description.

An enroute flight level change (a "switch") is encoded with the route text as a combination of two numbers, one for speed and one for altitude. Each number is prefixed with an indicator of unit. Two most common unit indicators are N for knots (nautical miles per hour) and F for flight level (100s of feet above standard altitude sea level). For example, N0400F340 means 400 knots at Flight Level 340 (34.000 ft).

"Switches", or in other words pairs of speed and altitude values can exist in route text at the beginning of it, setting the initial cruise speed & altitude, or behind a specific waypoint name, separated by "/", indicating that these new values will be valid from this particular waypoint onwards.

While switches are an industry standard, vroute uses one non-standard extension. Since vroute's flight plans are often valid for a range of flight levels,